Curious to see where you would have run into other Foursquare users? Want to
reminiscence about all the places you've been together with someone? Are you
Ted from How I Met Your Mother, and now that you've finally met the
mother, you want to know what all your missed opportunities were?

Allow Intersquares to connect to your Foursquare account, and you'll find
out (here's a screenshot of what an
intersection looks like, or
the screencast
showing the whole app in action).